Spatial variation of superconducting properties of Gd123 Bulk superconductors with magnetic particles addition

Magnetic flux trapping and the homogeneity of the flux pinning are essential problems in the practical application of high-temperature superconductors. We have conducted study on the role of addition of soft magnetic Fe-B alloy particles contribute to the enhancement of the critical current density (J_c) under wide-range of magnetic field. Magnetic flux trapping was enhanced in Gd123 bulk superconductor with suitable amount of magnetic particles addition. In addition, it can be effective as pinning center enhance the J_c of the bulk in both the a–b growth sector and the c-growth sector under magnetic field. However, the T_c of the Gd123 bulk was decreased obviously by addition of magnetic particles. The study on the spatial variation of superconducting properties indicates that the performance of the upper part of the bulk is better than the bottom. By comparing the superconducting properties of the Gd123 bulk with magnetic particles addition and without magnetic particles addition, we concluded that there is a trace of the formation of homogeneous pinning properties in the magnetic particles addition Gd123 bulk.     

强磁场对熔融织构GdBCO块材中Gd_2BaCuO_5颗粒的取向作用

在GdBCO超导块材的熔融织构生长过程中施加10T强磁场,通过SEM研究强磁场对Gd2BaCuO5(Gd211)颗粒的影响.研究发现棒状Gd211颗粒在强磁场的作用下发生了择优取向——Gd211颗粒的长边与磁场方向平行.延长样品处于熔融状态的时间,发现Gd211颗粒的取向性明显增加.通过分析熔融织构的相变过程,结合磁取向能的公式,讨论了取向性增加的原因.同时,探讨了Gd211颗粒的择优取向对样品的超导性能产生的影响.     

Enhancement of critical current density in Gd123 bulk superconductor doped with magnetic powder

We studied the materials processing to grow high performance magnet with doping of magnetic powder as candidature of pinning centers. The magnetic powder was doped into Gd123 matrix and grew into single domain bulk in air. With doping magnetic powder in Gd123 bulk superconductors, the flux pinning was enhanced and the underlying mechanism was discussed. The experimental results showed that suitable amount of magnetic powder inclusion can be effect as pinning center to enhance the J_c of the bulk both in low magnetic field and high magnetic field, which was different from the bulk doped with paramagnetic nano-scaled particles. The T_c of the bulk was suppressed obviously with the content of the magnetic powder increasing, form 93.0 K without doping decrease to 90.5 K with 0.12 wt.% doping. This is consistent with that the magnetic impurities generally suppress superconductivity. We report the detailed results from the viewpoint of practical application to increase the integrated flux trapped inside the bulk.     

Pinning effect of different shape second-phase particles on grain growth in polycrystalline: numerical and analytical investigations

The pinning effect of different shape second-phase particles on the grain growth in polycrystalline structures is numerical simulated by the phase-field method. Simulation results indicate that the average grain size is highly dependent on the shape and distribution of the second-phase particles, and the shape effect of particles on grain growth restraining is enhanced with increasing numbers of particles. In order to discuss the relation between the constraint grain growth and the second-phase particles, pinning forces induced by different shape particles are theoretically calculated via the Zener pinning theory. The calculated pining forces indicate that the maximum pinning force is highly dependent on the contact mode between grains and particles, and the distance between particles has a significantly influence on the pinning forces. Therefore, controlling the shape and distributions of second-phase particles in polycrystalline metals or ceramics might be an efficient way to achieve materials with specified microstructures.     

Photoemission study of Gd metal overlayers on a GaAs(110) surface

We have studied the chemical reactions of Gd metal on an in situ cleaved GaAs(110) surface by photoemission spectroscopy of Ga 3d and As 3d core-levels as well as the Gd 4f level on- and off-resonance valence band using synchrotron radiation. We find that the Fermi-level pinning is completed before 0.13 ML coverage, and the deposited Gd atoms start to react with the GaAs substrate at a very low coverage (critical coverage < 0.067 ML). As more Gd atoms are deposited, they form stable compounds with As atoms which are then trapped in the relatively narrow interfacial layer of thickness less than about 3.3 ML, while Ga atoms diffuse out towards the surface and eventually become metallic. The thickness of the GdGa intermixed layers is estimated to be about 6.7 ML, which is somewhat greater than that for a interface.     

New type of vortex pinning structure effective at very high magnetic fields

We report on a new type of correlated nanometer-scale pinning structure observed in a melt-processed (Nd0.33Eu0.38Gd0.28)Ba(2)Cu(3)O(y) (NEG-123). It consists of NEG/Ba-rich clusters in the stoichiometric NEG-123 matrix forming a lamellar array with a period of a few nanometers. These lamellas appear within regular twins, thus representing their fine substructure-sometimes straight, sometimes wavy. This new material structure correlates well with the significant enhancement of pinning at high fields, represented by irreversibility field above 14 T at 77 K (B parallel c). We believe that the new pinning medium enables one to significantly broaden the limits for high-field applications.     

Phase transitions of colloidal monolayers in periodic pinning arrays

We study the phase behavior of two-dimensional paramagnetic colloidal systems on square pinning arrays, the latter being created by a holographic optical tweezer technique. When the particle interaction strength is decreased, a transition from an incommensurate to a commensurate solid is observed. At even smaller pair potentials, the interstitial particles start to melt, whereas the particles at the substrate pinning sites are still localized. Our results are in good agreement with recent numerical studies on vortex melting in periodic pinning arrays.     

Effects of charge and dipole on flatband voltage in an MOS device with a Gd-doped HfO_2 dielectric

Gd-doped HfO2 has drawn worldwide interest for its interesting features.It is considered to be a suitable material for N-type metal-oxide-semiconductor(MOS)devices due to a negative flatband voltage(Vfb)shift caused by the Gd doping.In this work,an anomalous positive shift was observed when Gd was doped into HfO2.The cause for such a phenomenon was systematically investigated by distinguishing the effects of different factors,such as Fermi level pinning(FLP),a dipole at the dielectric/SiO2interface,fixed interfacial charge,and bulk charge,on Vfb.It was found that the FLP and interfacial dipole could make Vfbnegatively shifted,which is in agreement with the conventional dipole theory.The increase in interfacial fixed charge resulting from Gd doping plays a major role in positive Vfbshift.     

Gd2Ba4CuWOx掺杂对单畴GdBCO超导块材性能的影响

本文采用顶部籽晶熔融织构方法(TSMTG)制备出了具有不同纳米Gd2Ba4CuWOx(GdW2411)掺杂量的系列单畴GdBCO超导块材,并研究了GdW2411的掺杂量对其微观形貌以及磁悬浮力大小的影响.研究结果表明,通过添加GdW2411粒子,可以成功的在单畴GdBCO超导块材中引入纳米GdW2411磁通钉扎中心,其粒径约在50～200nm之间;随着GdW2411掺杂比例的增加,纳米粒子的密度逐渐变大,粒度也有所变大;GdBCO超导块材磁悬浮力的大小与GdW2411掺杂量密切相关,只有当GdW2411的掺杂比例达到最佳值时,样品的磁悬浮力才达到最大.这些结果对进一步提高GdBCO超导块材性能具有重要的指导意义.     

Directional Mode-Locking of Dynamics of Two-Dimensional Superparamagnetic Colloidal System in a Periodic Pinning Array

Using Langevin simulations,we study numerically the directional mode-locking of the dynamics of twodimensional superparamagnetic colloidal system in a periodic pinning array.When the colloidal particles are initially commensurate with the pinning sites,there appear mode-locking steps in the average velocity of colloidal particles along certain directions of the external driving force.With an increase in the pinning strength,the width of the step increases linearly but the velocity at the step decreases parabolically.     

AC susceptibility enhancement studies in magnetic systems

Enhancement of AC susceptibility has been observed for typical ferromagnets (Gd), reentrant spin glasses like (Fe_1.5Mn_1.5Si) and canted spin systems (Ce(Fe_0.96Al_0.04)₂). The data have been interpreted with the help of a simulation model based on dry friction-like pinning of domain walls for systems having ferromagnetic domain structures. A strong pinning mechanism appears in the reentrant spin glass like and canted spin systems at low temperatures in addition to the intrinsic one in the ferromagnetic phase. The temperature variation of the pinning potential has been given qualitatively for the reentrant spin glass like systems.     

Phase-field study of the second phase particle effect on texture evolution of polycrystalline material

The second phase particle effect on texture evolution of polycrystalline material is studied through phase-field method. A unique field variable is introduced into the phase-field model to represent the second phase particles. Elastic interaction between particles and grains is also considered. Results indicate that in the presence of second phase particles the average particle diameter turns smaller than in the absence of these particles and retards texture formation by pinning effect. The second phase particles change the strain energy profile, which tremendously influences the pinning effect.     

Grain boundary curvature and grain growth kinetics with particle pinning

Second-phase particles are used extensively in design of polycrystalline materials to control the grain size. According to Zener’s theory, a distribution of particles creates a pinning pressure on a moving grain boundary. As a result, a limiting grain size is observed, but the effect of pinning on the detail of grain growth kinetics is less known. The influence of the particles on the microstructure occurs in multiple length scales, established by particle radius and the grain size. In this article, we use a meso-scale phase-field model that simulates grain growth in the presence of a uniform pinning pressure. The curvature of the grain boundary network is measured to determine the driving pressure of grain growth in 2D and 3D systems. It was observed that the grain growth continues, even under conditions where the average driving pressure is smaller than the pinning pressure. The limiting grain size is reached when the maximum of driving pressure distribution in the structure is equal to the pinning pressure. This results in a limiting grain size, larger than the one predicted by conventional models, and further analysis shows consistency with experimental observations. A physical model is proposed for the kinetics of grain growth using parameters based on the curvature analysis of the grain boundaries. This model can describe the simulated grain growth kinetics.     

Three-dimensional finite-element simulation of Zener pinning dynamics

The Zener pinning dynamics of a moving boundary interacting with one or more particles is described by a three-dimensional (3D) finite-element model. The model, based upon a variational formulation for boundary motion by viscous drag, is solved by a finite-element method to obtain the velocity at each node of triangular linear elements on the grain boundary. It is first applied to relatively simple and validated cases, for which analytical and numerical results are available. These cases correspond to an axisymmetrical geometry, in which the grain boundary interacts with a centred particle. A simple analytical pinning criterion is derived from these simulations. The model is then applied to general 3D cases, in which the grain boundary interacts with arbitrarily localized and sized particles. The aim of these 3D simulations is to quantify the influence of the position and the number of particles on the average grain-boundary velocity. It is shown, for example, that the drag effect is enhanced when the particle, or the cluster of particles, is off-centre and that pinning is less efficient with several particles than with a single particle producing the same Zener force.     

Electrical resistivity and magnetic investigations of the orthorhombic Tb(Ni,Cu)2 system

The orthorhombic Tb(Ni, Cu)₂ and Gd(Ni, Cu)₂ systems (CeCu₂ structure) are closely similar according to electrical resistivity and magnetic results. The Tb(Ni_xCu_1?x)₂ system presents a transition from antiferromagnetism (AF) for x ? 8% Ni to ferromagnetism (FM) for x &#62; 8% Ni. The CeCu₂ structure becomes unstable for x &#62; 45% Ni. The AF samples show metamagnetism at 4.2 K with critical fields. Hysteresis, which occurs for all samples at 4.2 K, is attributed to intrinsic pinning due to large anisotropy where the mechanism for the AF range is analogous to intrinsic pinning of narrow domain walls in FM samples.Spin disorder resistivity measurements show a discontinuity at the AF-FM transition composition for both the Tb and Gd systems. This is due to a step up of the residual resistivity at 4.2 K as a result of AF ordering. This interpretation is confirmed by applying a magnetic field to destroy the AF ordering.     

纳米微粒BaFe12O19掺杂对单畴超导块材GdBa2Cu3O7-δ性能的影响

磁通钉扎性能对GdBa₂Cu₃O_7-δ超导块材的实际应用具有重要的影响, 而引入合适的第二相粒子可以改善GdBa₂Cu₃O_7-δ 超导块材的磁通钉扎性能.本文采用顶部籽晶熔融织构法成功地制备出纳米微粒BaFe₁₂O₁₉(<100 nm)掺杂的超导块材, 样品的最终组分为Gd123+ 0.4 Gd211+ x BaFe₁₂O₁₉ (x=0, 0.2 mol%, 0.4 mol%, 0.8 mol%)+ 10 wt%Ag₂O+ 0.5 wt%Pt. 通过研究不同掺杂量的BaFe₁₂O₁₉微粒对GdBa₂Cu₃O_7-δ 超导块材微观结构和超导性能的影响, 结果表明当掺杂量为0.2 mol%时, 样品的临界电流密度几乎在整个外加磁场下都有明显的提高.在零场下, 临界电流密度达到5.5× 10⁴ A/cm². 纳米微粒BaFe₁₂O₁₉不仅可以保持掺杂前的化学组成, 作为有效的钉扎中心存在于超导块材中, 并且能够改善Gd₂BaCuO₅粒子的分布和细化Gd₂BaCuO₅粒子, 使Gd₂BaCuO₅粒子的平均粒径由未掺杂时的1.4 μ m减小到掺杂后的0.79 μ m, 进而提高了超导块材的临界电流密度和俘获磁场, 明显提高了GdBa₂Cu₃O_7-δ 超导块材的超导性能.临界温度T_C也有所提升, 并能够维持在92.5 K左右. 该结果为进一步研究纳米磁通钉扎中心的引入并改善GdBa₂Cu₃O_7-δ 超导块材的性能有着重要的意义.     

BaO掺杂对单畴GdBCO超导块材性能的影响(二)

为了有效地抑制Gd BCO超导块材在生长过程中出现的Gd/Ba替换现象,在前期工作的基础上,本文采用顶部籽晶熔渗生长工艺,通过在固相先驱粉中添加不同含量的Ba O粒子成功地制备出了一系列高性能的单畴Gd BCO超导块材,并且对样品的微观形貌以及临界电流密度进行了研究和分析.结果表明,随着BaO掺杂量的增加,样品中的Gd_(1+x)Ba_(2-x)Cu_3O_(7-δ)固溶体(Gd123ss)相呈现出减少的趋势,并生成了纳米量级的Gd123ss,这对GdBCO超导样品中存在的Gd/Ba替换起到了很好的抑制作用,使得样品中的GdBa_2Cu_3O_(7-δ)(Gd123)超导相有所增加;同时当样品中BaO的添加量在2 wt%—4 wt%之间时,样品的临界电流密度在一定程度上得到了有效提高.     

Effects of tin-compounds addition on Jc and microstructure for YBCO films

Tin-compounds were doped into YBa₂Cu₃O_7−δ (YBCO) films as pinning centers to enhance J_c in magnetic fields. YBCO films were grown by a metal organic deposition process using tri-fluoroacetates starting solutions. Tin-acetylacetonate salts were then dissolved into the starting solution as pinning centers. J_c of the YBCO films with tin-acetylacetonates were enhanced in all magnetic field angles. Transmission electron microscopy (TEM) and TEM-EDS (Energy Dispersive X-ray Spectroscopy) observations revealed the existence of tin-compound particles with the size of about 30 nm in the YBCO film. These nano particles were distributed randomly in the film and were considered to act as 3-dimentional pinning centers.     

Gadolinium oxide: a protoype agent for contrast enhanced imaging of the liver and spleen with magnetic resonance

Gd2O3 particles (less than 2 microns) in suspension were evaluated as a potential contrast agent for liver-spleen imaging with magnetic resonance. The agent was administered IV to rabbits in doses ranging from 10 to 120 mumol/kg and the tissues removed after sacrifice for in vitro T1 and T2 analysis. The temporal response was determined in liver and spleen samples of rabbits given a fixed dose (60 mumol/kg) and sacrificed at intervals from 15 min to 60 hr later. Documentation of the subanatomic location of Gd2O3 particles in tissue was accomplished by electron microscopy and x-ray dispersion microanalysis. T1 weighted images were obtained at 0.12T on a prototype resistive scanner. The liver, spleen, and lung relaxation times are very responsive to Gd2O3 IV and the effect is dose related. A peak effect is observed between 3-7 hr after injection and relaxation times may normalize by 60 hr. By electron microscopic and x-ray analysis, Gd2O3 is most prominently found in the hepatic and splenic sinusoids. The images show marked enhancement of liver and splenic tissues, aiding in the clear delineation of these tissues from neighboring structures.     

Cu位La、 Gd掺杂YBa2Cu3O7-δ结构和磁通钉扎的比较

多晶样品YBa2Cu3-xLaxO7-δ(0≤x≤0.15)和YBa2Cu3-xGdxO7-δ(0≤x≤0.15)通过固相反应法制备得来.通过结构参数分析,我们发现La首先替代Cu(1)位,然后开始部分替代Ba,而Gd开始替代Cu(2)位然后替代Cu(1)位.YBa2Cu3-xLaxO7-δ和YBa2Cu3-xGdxO...